Lead author Stephanie Pierce, of the University of Cambridge's Department of Zoology, was quoted as saying in a press release, "The results of this study force us to re-write the textbook on backbone evolution in the earliest limbed animals."

Pierce and her colleagues bombarded 360-million-year-old early fossils for four-legged animals with high-energy synchrotron radiation. The resulting high resolution x-ray images allowed the researchers to reconstruct the backbones of the extinct animals in exceptional detail.

Today, all four-limbed animals (technically known as tetrapods) possess a backbone. It is formed from many bony segments, called vertebrae, all connected in a row from head to tail/rear end.

Backbone of many parts

Unlike the backbone of living tetrapods like humans, in which each vertebra is composed of only one bone, early tetrapods had vertebrae made up of multiple parts.

Pierce says, "For more than 100 years, early tetrapods were thought to have vertebrae composed of three sets of bones — one bone in front, one on top, and a pair behind. But, by peering inside the fossils using synchrotron x-rays, we have discovered that this traditional view literally got it back-to-front."

The team of scientists discovered that what was thought to be the first bone - known as the intercentrum — is actually the last in the series.

"By understanding how each of the bones fit together we can begin to explore the mobility of the spine and test how it may have transferred forces between the limbs during the early stages of land movement," says Pierce.

Aside from deducing that Ichthyostega and its ilk moved like seals, the researchers also discovered a string of bones that extended down the middle of its chest.

Co-author Jennifer Clack explains, "These chest bones turned out to be the earliest evolutionary attempt to produce a bony sternum. Such a structure would have strengthened the ribcage of Ichthyostega, permitting it to support its body weight on its chest while moving about on land."

The next phase of research will further investigate how the backbone aided locomotion in these early four-limbed animals.